As components of living things in the earth, there are materials showing various characteristics caused by their asymmetric structures, and it is generally known that among physiologically active materials having an asymmetric structure, those useful for a human being are frequently of specific antipodes, such tendency is particularly remarkable in the fields of, for examples, medicaments, agricultural chemicals, pheromones, and food additives, and that when they are a racemic mixture, the effect is greatly reduced, or a negative function appears. For example, JP-A-63-44544 describes that: "In the present science, while there is no single theory which can elucidate the phenomenon of the sense of smell, it has frequently been experimentally clarified that only one optically active substance of a certain compound has desired odor properties and that while a racemate thereof has similar smell character characteristics, its aroma intensity is not more than a half thereof at the most" (the term "JP-A" as used herein means an "unexamined Japanese patent application").
This is not exceptional in the field of aroma chemicals. For example, for the odor of peppermint, L-menthol is useful, for the odor of cumin, d-carvone is useful, and for the odor of grapefruit, d-nootkatone is useful, respectively, but they each has a different odor from other optical antipodes, or the odor is stronger than other optical antipodes. As recent examples, such phenomena are observed in .alpha.-ionone, hydroxycitronellal, rose oxide, etc. (see, Kagaku Sosetsu, 14 (1976), "Aji To Nioi No Kagaku (Chapter 6). Therefore, the development of optically active aroma chemicals is significant for the development of new odors or effective optical antipodes like the development of new aroma chemicals.
Ethyl 2,2,6-trimethylcyclohexanecarboxylate is a known compound already described in Chem. Ber., Vol. 102, 709-711 (1969), but its odor and value as an aroma chemical are not described therein. Also, since ethyl 2,2,6-trimethylcyclohexanecarboxylate has asymmetric carbon atoms in the molecule, while it is expected that two mirror-image isomers of a (+) isomer and a (-) isomer exist, the ethyl 2,2,6-trimethylcyclohexanecarboxylate described in the foregoing literature is a racemic mixture, and neither report regarding the individual synthesis of these isomers nor report about the properties of the isomers.
Also, as an analogue of the 2,2,6-trimethylcyclohexanecarboxylic acid ester compound known as an aroma chemical, a mixture of ethyl 2-ethyl-6,6-dimethylcyclohexanecarboxylate and ethyl 2,3,6,6-tetramethylcyclohexanecarboxylate is disclosed in JP-B-2-62542 (the term "JP-B" as used herein means an "examined published Japanese patent application"), but the properties of the individual compounds and the odor characteristics as a mixture are not shown therein.
With respect to racemic 2,2,6-trimethylcyclohexanecarboxylic acid derivatives, investigations have been made about the production process of a violet odor [see, Helv. Chem. Acta, 31, 134 (1948)]. Also, ethyl (1R,6S)-2,2,6-trimethylcyclohexanecarboxylate according to the present invention (hereinafter sometimes referred to simply as "the compound of the present invention") is a very useful aroma chemical having a fruit-like and floral odor.
The inventors carried out the process shown by the following reaction scheme for producing (1R,6S)-2,2,6-trimethylcyclohexanecarboxylic acid. That is, using optically active (3S)-7-methoxycitronellal as a raw material, enol acetate was first formed and cyclized to provide (1R,6S)-2,2,6-trimethylcyclohexanecarbaldehyde, which was then oxidized to provide (1R,6S)-2,2,6-trimethylcyclohexanecarboxylic acid, and this acid was esterified to synthesize the compound of the present invention, i.e., ethyl (1R,6S)-2,2,6-trimethylcyclohexanecarboxylate. ##STR2##
As an example of obtaining an optically active carboxylic acid by recrystallizing a carboxylic acid as a diastereomer salt of an asymmetric amine to undergo optical resolution, there are known a process of producing p-isopropyl .alpha.-methyldihydrocinnamate as described in JP-A-55-27166 and a process of producing p-tert-butyl .alpha.-methyldihydrocinnamate as described in JP-A-55-35459.
Recently, with the diversity of various fragrance cosmetics and hygienic materials, a new demand which has not been known in conventional fragrance cosmetics and hygienic materials has been being increased; the development of aroma chemical substances having strong diffusibility, specific fragrance quality, high preference, long lasting, good stability, and high safety has been required; and in particular, aroma chemical materials having a floral and fruit-like odor, which can satisfy the aforesaid requirements, are insufficient.
Accordingly, the present invention is aimed to provide an aroma chemical material or aroma chemical composition capable of satisfying the aforesaid requirements and giving a floral and fruit-like odor.
In general, it is known that in aroma chemical compositions, even a slight difference in top note, etc. of a single aroma chemical sometimes changes the fragrance quality as a product. In particular, in a high-class perfume, etc., a delicate difference of a single aroma chemical determines the propriety as the perfume. In particular, in the case of a single aroma chemical giving a floral and fruit-like odor, delicate odor factors such as naturalness and freshness are important as a value of the aroma chemical.
In regard to the relation of the steric structure and odor of the compound of the present invention, the inventors have recognized that in the geometric isomers (i.e., the cis-form and the trans-form), the trans-form is superior to the cis-form and that in the optical isomers of the trans-form [i.e., the (1R,6S)-form and the (1S,6R)-form], the (1R,6S)-form is superior to the (1S,6R)-form. In other words, the higher the ratio of the trans-form to the cis-form and the higher the optical purity, the more excellent the odor and the higher the value as an aroma chemical.
However, in the component composition of ethyl (1R,6S)-2,2,6-trimethylcyclohexanecarboxylate which was obtained first by the inventors, the content of the (1R,6S)-form as the trans-form was about 90%, and the content of the (1S,6S)-form as the cis-form was about 10%. Also, in regard to the optical purity, when not subjected to the optical resolution, while the optical purity of the product depends on the optical purity of a raw material, since commercially available (3S)-7-methoxyoitronellal as the raw material has an optical purity of 98% ee, the optical purity of the product is not higher than the foregoing optical purity. That is, there was a room for an improvement in the ethyl (1R,6S)-2,2,6-trimethylcyclohexanecarboxylate first obtained by the inventors.
Accordingly, in the component composition of ethyl (1R,6S)-2,2,6-trimethylcyclohexanecarboxylate, it was important to develop a process of producing geometrically and optically pure ethyl (1R,6S)-2,2,6-trimethylcyclohexanecarboxylate.